1. Field of the Invention
The present invention relates to a pulse detection device using a piezoelectric element and a method of manufacturing the pulse detection device.
2. Description of the Related Art
A pulse in a living body contains important information for diagnosis of a sickness. In recent years, studies have been conducted on application of a system in which a portable pulse detection device is worn about an arm of a patient at a medical office of a medical institution, e.g., a hospital, and data on a pulse detected from the patient, which is transmitted from the portable pulse detection device, is received on the hospital side to grasp the condition of the patient. Use of a piezoelectric element in a pulse detection device is advantageous in terms of the effect of reducing the device in size and in weight. The development of pulse detection devices, including those suitably applicable to the above-described system, is being advanced.
FIGS. 19 and 20 show a conventional pulse detecting device 100 using a piezoelectric element. As shown in FIG. 19, the pulse detection device 100 has two piezoelectric elements 110 and 120 fixed by being embedded in a resin 130 (or gel). FIG. 20 is a side view of the pulse detection device 100 shown in FIG. 20. The piezoelectric elements 110 and 120 are fixed while being spaced apart by a distance g. Further, the fixing step is controlled such that the thickness t between the piezoelectric elements 110 and 120 and a surface 150 to be brought into contact with a skin is set to a predetermined value.
In each of the piezoelectric elements 110 and 120, metal electrodes (not shown) are formed on two surfaces opposite from each other in the direction of thickness. Probes (terminals, leads, or the like) (not shown) for application of a drive voltage are connected to both the electrodes of the piezoelectric element 110 while probes (not shown) for outputting a voltage signal are connected to the upper and lower electrodes of the piezoelectric element 120.
This pulse detection device 110 is used to detect a pulse of a patient at the time of medical examination in a hospital. More specifically, when a drive voltage is applied to both the electrodes of the piezoelectric element 110, the piezoelectric element 110 is excited to generate ultrasound, which is transmitted into the living body through the resin 130. The ultrasound transmitted into the living body is reflected by a bloodstream in the living body. The reflected ultrasound is received by the piezoelectric element 120 through the resin 130.
At this time, a difference is caused between the frequency of the ultrasound transmitted by the piezoelectric element 110 and the frequency of the ultrasound received by the piezoelectric element 120 by the Doppler effect of the bloodstream. Since the speed of the bloodstream changes in synchronization with pulsation, a pulse in the living body is detected from changes in the frequency of the ultrasound.
In the above-described pulse detection device using the piezoelectric elements, there is a need to precisely place the ultrasound transmitting piezoelectric element 110 and the ultrasound receiving piezoelectric element 120 for the purpose of improving the ultrasound receiving sensitivity. The pulse detection sensitivity changes largely with respect to the spacing g between the two piezoelectric elements 110 and 120. An optimum range of this spacing is from 0.1 to 0.5 mm. The sensitivity also changes largely with respect to the thickness t of the block of the resin 130. For example, if the piezoelectric elements 110 and 120 are driven at 9.0 MHz, an optimum value of the thickness t is about 140 xcexcm.
However, it is difficult to precisely place the piezoelectric elements 110 and 120 of the above-described pulse detection device 100, because the manufacturing process of the pulse detection device 100 uses the step of pouring the resin 130 after placing the two piezoelectric elements 110 and 120 at predetermined positions and there is a possibility of occurrence of changes in the positions and the angles of the placed piezoelectric elements when the resin is poured.
Therefore, there is a possibility of the conventional pulse detection devices 100 being manufactured with considerable variation in quality. In the step of forming the piezoelectric element by pouring the resin 130, it is difficult to control the thickness h of the resin 130 in accordance with the desired thickness, so that there is a possibility of occurrence of considerable variation in sensitivity.
Further, because of the need to apply a voltage between the two surfaces opposite in the thickness direction of each of the piezoelectric elements 110 and 120 embedded in the resin 130, the step of attaching thin wires or the like to the two surfaces of the piezoelectric elements 110 and 120 is required before the step of pouring the resin 130. Therefore, it is difficult to suitably place the piezoelectric elements 110 and 120, and the number of steps is increased. Thus, it is difficult to manufacture the pulse detection device.
In view of the above-described problems, an object of the present invention is to provide a pulse detection device in which an ultrasound transmitting piezoelectric element and an ultrasound receiving piezoelectric element are precisely placed to effectively limit variation in quality, and a method of manufacturing the pulse detection device.
Another object of the present invention is to provide a pulse detection device which is formed by using a base plate having a prescribed thickness to have an optimum pulse detection sensitivity, instead of resin, and which can be manufactured with improved reproducibility.
Still another object of the present invention is to provide a pulse detection device in which wiring for application of a voltage to piezoelectric elements is provided on a supporting base plate to easily manufacture the pulse detection device.
A further object of the present invention is to provide a pulse detection device having an improved pulse detection sensitivity.
To achieve the above-described objects, according to an aspect of the present invention, there is provided a pulse detection device comprising a transmitting piezoelectric element (e.g., transmitting piezoelectric element 41 shown in FIG. 4) excited in accordance with a drive signal to generate ultrasound and to transmit the ultrasound into a living body, a receiving piezoelectric element (e.g., receiving piezoelectric element 42 shown in FIG. 4) for receiving reflected waves of the ultrasound transmitted into the living body and reflected by a bloodstream in the living body, and for converting the reflected waves into an electrical signal, a detection section (e.g., processing computation section 31 shown in FIG. 3) for detecting a pulse from the ultrasound generated by the transmitting piezoelectric element and the reflected waves received by the receiving piezoelectric element, and a transmitting/receiving base plate (e.g., transmitting/receiving base plate 43) having the transmitting piezoelectric element and the receiving piezoelectric element placed and fixed on its one surface, the other surface of the transmitting/receiving base plate being brought into contact with the living body.
In the thus-constructed pulse detection device, both the transmitting piezoelectric element and the receiving piezoelectric element are placed and fixed on the transmitting/receiving base plate. Therefore, these piezoelectric elements can be placed with accuracy in accordance with a design.
There is no functional problem since the ultrasound generated by the transmitting piezoelectric element is transmitted into the living body through the transmitting/receiving base plate, and the waves reflected by the bloodstream in the living body propagate from the living body to the receiving piezoelectric element through the transmitting/receiving base plate.
According to the present invention, therefore, it is possible to provide a pulse detection device with a reduced possibility of occurrence of variation in quality. It is also possible to improve the pulse detection sensitivity of the pulse detection device.
The acoustic impedance of the transmitting/receiving base plate is set to an intermediate value between the acoustic impedance of each of the piezoelectric elements and the acoustic impedance of the living body.
The acoustic impedance of the transmitting/receiving base plate is set to an intermediate value between the acoustic impedance of each of the piezoelectric elements and the acoustic impedance of the living body to enable the ultrasound generated by the transmitting piezoelectric element to be efficiently transmitted into the living body without being reflected at the interface between the transmitting/receiving base plate and the living body. Further, it is possible to receive the reflected waves by the pule in the living body with the receiving piezoelectric element with high sensitivity without being reflected at the interface.
A glass base plate having a thickness of about xc2xc of the wavelength of the ultrasound generated by the transmitting piezoelectric element may be used as the transmitting/receiving base plate. The amount of reflection of the ultrasound at the interface between the glass base plate and the living body can be reduced by using such a glass base plate, thereby enabling efficient transmission of the ultrasound into the living body and enabling the receiving piezoelectric element to receive the reflected waves with high sensitivity.
Further, a resin layer (e.g., resin layer 49 shown in FIG. 8) may be provided on the other surface of the transmitting/receiving base plate. The material of the resin layer may be selected to optimize, according to use of the pulse detection device, the characteristics of the surface to be brought into contact with the living body.
For example, an epoxy-based resin is used to form the resin layer. Since the acoustic impedance of the epoxy-based resin is an intermediate value between the acoustic impedance of the transmitting/receiving base plate and the acoustic impedance of the living body, the amount of reflection of the ultrasound at the interface between the transmitting/receiving base plate and the living body can be further reduced by using the epoxy-based resin layer, thereby enabling efficient propagation of the ultrasound.
If a silicone-based resin, for example, is used to form the resin layer, the closeness of contact between the transmitting/receiving base plate and the living body can be improved. Correspondingly, the amount of intrusion of air at the interface between the transmitting/receiving base plate and the living body is reduced and the attenuation of the ultrasound is thereby reduced. As a result, the efficiency of propagation of the ultrasound is improved. The silicone-based resin is favorable in terms of compatibility with the living body and can be used by being maintained in intimate contact with the skin of the living body without any considerable risk of affecting the skin.
The above-described pulse detection device may constructed in such a manner that a groove (groove 50a shown in FIG. 9) is formed in a portion of the transmitting/receiving base plate, and the transmitting piezoelectric element and the receiving piezoelectric element are placed on the transmitting/receiving base plate on the opposite sides of the groove.
According to this arrangement, the ultrasound generated by the transmitting piezoelectric element is reflected and attenuated at the groove between the transmitting piezoelectric element and the receiving piezoelectric element on the transmitting/receiving base plate. Thus, the possibility of the ultrasound propagating through the transmitting/receiving base plate to be directly received by the receiving piezoelectric element is reduced. Consequently, noise can be reduced and the reliability of the pulse detection device can be improved.
Alternatively, the transmitting/receiving base plate may be divided into two. The transmitting piezoelectric element may be placed on one of the divided transmitting/receiving base plates (e.g., transmitting/receiving base plate 51 shown in FIG. 10), and the receiving piezoelectric element may be placed on the other divided transmitting/receiving base plate (e.g., transmitting/receiving base plate 52 shown in FIG. 10). In such a case, the ultrasound generated by the transmitting piezoelectric element does not propagate directly to the receiving piezoelectric element. Thus, noise can be reduced and the reliability of the pulse detection device can be improved.
The other surface of the transmitting/receiving base plate (e.g., transmitting/receiving base plate 53 shown in FIG. 11) may be formed so as to be slanted relative to the surface on which the piezoelectric elements are placed. For example, the two surfaces of the transmitting/receiving base plate are formed so as to be not parallel but tapered, thereby enhancing the Doppler effect of the bloodstream in the living body to increase the difference between the frequency of the ultrasound generated by the transmitting piezoelectric element and the reflected waves received by the receiving piezoelectric element. Consequently, the pulse detection sensitivity of the pulse detection device is improved.
A supporting base plate (e.g., supporting base plate 44 shown in FIG. 4) for supporting the transmitting piezoelectric element and the receiving piezoelectric element positioned on the transmitting/receiving base plate may be provided.
If such a supporting base plate is provided, the strength of the pulse detection device against an impact externally applied can be improved, thereby improving the durability of the device.
Further, the supporting base plate 44 provided in the pulse detection device is effective in preventing leakage of ultrasound.
A display section may be provided to display a pulse detected by the detection section.
A belt (e.g., band 5 shown in FIG. 1) for enabling the pulse detection device to be worn about a wrist may be provided to enable the living body to easily carry the pulse detection device.
The transmitting piezoelectric element and/or the receiving piezoelectric element, and the transmitting/receiving base plate may be joined to each other by intermetallic bonding.
Intermetallic bonding is a method of joining two metals by maintaining the two metals in contact with each other, and pressing and heating the metals to cause thermal diffusion of metal atoms between the metals.
In the thus-constructed pulse detection device, the transmitting/receiving base plate and each of the transmitting piezoelectric element and the receiving piezoelectric element are joined by intermetallic bonding. The attenuation of ultrasound at the joint surface in the case of joining by intermetallic bonding is smaller than that in the case of joining by using an adhesive. Thus, efficient propagation of ultrasound can be achieved.
A sealing material such as a silicone resin may be provided between the transmitting/receiving base plate and the supporting base plate. In the thus-arranged pulse detection device, it is possible to prevent sweat or the like from attaching to the transmitting piezoelectric element and the receiving piezoelectric element during use and, hence, to prevent a reduction in sensitivity. The sealing material is provided such that a gap is set between the sealing material and each of the transmitting piezoelectric element and the receiving piezoelectric element, so that external vibration cannot easily propagate to the piezoelectric elements through the sealing material. Thus, it is possible to prevent sweat or the like from entering the pulse detection device and to further improve the effect of preventing a reduction in sensitivity.
The pulse detection device may also be constructed in such a manner that a channel is formed in the transmitting/receiving base plate and the piezoelectric elements are placed in the channel, thereby enabling the transmitting/receiving base plate and the supporting base plate to be fixed to each other in addition to a joining surface between the transmitting and receiving piezoelectric elements and the transiting/receiving base plate and a joining surface between the transmitting and receiving piezoelectric elements and the supporting base plate. Thus, durability of the device can be improved. Further, the thickness of the portion of the transmitting/receiving base plate remaining after the formation of the channel is set to an optimum value to enable efficient transmission and reception of ultrasound without changing the thickness of the entire transmitting/receiving base plate.
Feeding portions (transmitting/receiving base plate electrodes) to which an electrical signal is applied may be provided on one surface of the transmitting/receiving base plate on which the piezoelectric elements are placed, and feeding portions (supporting base plate electrodes) to be electrically connected to the transmitting/receiving base plate electrodes may be provided on the surface of the supporting base plate on which the piezoelectric elements are supported. The transmitting/receiving base plate electrodes and the supporting base plate electrodes are electrically connected to each other. In this manner, the amount of wiring for application of electrical signals to the transmitting piezoelectric element and the receiving piezoelectric element can be reduced.
According to another aspect of the present invention, there is provided a method of manufacturing a pulse detection device comprising the steps of forming wiring metal films on a transmitting/receiving base plate, and an electrode metal film on a transmitting piezoelectric element and on a receiving piezoelectric element, placing the transmitting piezoelectric element and the receiving piezoelectric element on the transmitting/receiving base plate such that the metal films are laid on each other, and joining the transmitting piezoelectric element and the receiving piezoelectric element to the transmitting/receiving base plate by using intermetallic bonding between the metal films to fix the transmitting piezoelectric element and the receiving piezoelectric element on the transmitting/receiving base plate and to establish an electrical connection between the transmitting/receiving base plate and each of the transmitting piezoelectric element and the receiving piezoelectric element.